Vaprisol is a newly released drug treatment for low sodium in the blood (also called "hyponatremia").

First, some background. Hyponatremia is a medical condition that can have many possible causes. For various reasons, the body retains too much water and the blood sodium is diluted. Some causes of hyponatremia include losing body fluids (from vomiting, diarrhea, or from diuretics); an abnormally high level of antidiuretic hormone, also called vasopressin (related to medical problems like strokes or lung cancer); and other conditions like cirrhosis, congestive heart failure, and protein in the urine.

Vaprisol has been approved to treat hyponatremia related to conditions in which the body has not lost body fluid. It is not indicated for conditions like vomiting, in which the body's fluid is low.

Here's how it works. Usually, a gland in the brain called the pituitary released antidiurtic hormone (vasopressin) in response to a high concentration of sodium in the blood serum. More vasopressin causes the kidneys to retain more water, which causes the blood sodium to go down as the extra sodium is diluted by more water. In many cases of hyponatremia, the level of vasopressin is abnormally high and the body therefore retains too much water, driving the sodium level down.

Vaprisol is an intravenous medication which blocks the effect of vasopressin on the kidney. It's usually given as a bolus of 20 mg followed by a slow infusion. By blocking the effects of vasopressin, vaprisol causes the kidney to eliminate water (a "water diuresis"). This eventually causes the serum sodium to go up as the blood sodium becomes less diluted.

Cardiologists are also interested in the possible future use of Vaprisol to treat low sodium related to congestive heart failure, but it is not yet approved for this indication.